Method and system of stimulation of nerve tissue with a sequence of spatially distributed resonant sub-threshold electrical stimuli

ABSTRACT

The present invention provides a method of stimulating an excitable tissue (e.g., in vitro, in vivo) with a primary electrical stimulus through a primary electrode at a primary stimulation frequency, to produce a propagating action potential in the excitable tissue. The invention is carried out by concurrently stimulating the excitable tissue with a secondary electrical stimulus through at least one secondary electrode at a secondary stimulation frequency. The primary and secondary stimulation frequencies are preferably different from one another. The secondary electrical stimulus preferably has an amplitude not more than one third that of the primary electrical stimulus. Preferably, propagation of the action potential in the excitable tissue is enhanced (e.g., when propagation of action potentials in the tissue is otherwise unstable, partially blocked, or fully blocked). Apparatus for carrying out the method is also described.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/988,728, filed Aug. 23, 2013, now allowed, which is a 35 U.S.C. §371national phase entry of PCT Application PCT/US2011/062807, filed Dec. 1,2011, and published in English on Jun. 28, 2012, as InternationalPublication No. WO 2012/087530, and which claims the benefit of U.S.Provisional Patent Application Ser. No. 61/424,758, filed Dec. 20, 2010,the disclosure of each of which is incorporated herein by reference inits entirety.

FIELD OF THE INVENTION

The present invention relates to functional electrical stimulation ofnerve tissue. The invention's method is based on a concept of non-linearresonance and may be implemented for acceleration and stabilization of awave propagating in nerve tissue with critically impaired excitation.

BACKGROUND OF INVENTION

Post-traumatic adjustment of pulse propagation in nerves with reducedexcitability is a challenging biomedical and technological problem. Inmany cases such adjustments can be achieved with surgical interventions.However, surgery does not necessarily restore nerve conductivity to itspre-trauma levels, therefore still impaired conduction may cause partialor complete muscular paralysis. In this case propagation of excitationwaves can be enforced only by applying external functional electricalstimulation (FES) using implantable [1] or surface stimulationelectrodes [2-4]. This method has been confirmed as an effective toolfor restoration of movement of paralyzed muscles in individuals withvariety of neurological impairments [5].

Computerized systems for control of FES can deliver sequences ofelectrical stimuli with different frequency, amplitude and duration [6].Commonly, these systems include variety of control units and electricalleads with multiple array or patch type stimulation electrodes. Theseelectrodes deliver programmable stimuli, which are designed to maximizethe effect of stimulation based on configuration of a particularstimulation field [7-10].

Successful FES process of functional restoration of muscle contractiondepends on ability of nerve tissue to adequately conduct actionpotential. It also depends on excitation-contraction coupling inneuromuscular junctions, which transmit nervous impulses to muscularfibers. If any of these steps is impaired muscle does not contractnormally. Usually, after severe neuromuscular injuries nerveconductivity is significantly reduced which, in turn, prevents thepassage of excitation waves through neuromuscular transmitters. Underthese circumstances propagation of excitation pulses is marginallystable and implementation of FES necessitates a significant increase offrequencies and amplitudes of functional electrical stimuli. The latest,instead of stabilization of propagation, can facilitate conductionblocks and may completely disrupt the process of training paralyzedmuscles.

Accordingly, there is a need for new methods, systems and apparatus forstimulating excitable tissues which treat the problem of conductioninstability and blockage therein.

SUMMARY OF THE INVENTION

A first aspect of the invention is a method of stimulating an excitabletissue (e.g., in vitro, in vivo) with a primary electrical stimulusthrough a primary electrode at a primary stimulation frequency, toproduce a propagating action potential in the excitable tissue. Theinvention comprises concurrently stimulating the excitable tissue with asecondary electrical stimulus through at least one secondary electrodeat a secondary stimulation frequency. The primary and secondarystimulation frequencies are preferably different from one another. Thesecondary electrical stimulus preferably has an amplitude not more thanone third that of the primary electrical stimulus. Preferably,propagation of the action potential in the excitable tissue is enhanced(e.g., when propagation of action potentials in the tissue is otherwiseunstable, partially blocked, or fully blocked).

In some embodiments, the primary stimulation frequency is from 1 or 5Hertz up to 12 or 20 Hertz (most preferably about 8 hertz).

In some embodiments, the secondary stimulation frequency is from 1 to 15percent greater than, or 1 to 15 percent less than, the primarystimulation frequency.

In some embodiments, the secondary electrical stimulus has an amplitudenot more than one tenth that of the primary electrical stimulus.

In some embodiments, the at least one secondary electrode comprises aplurality of secondary electrodes (e.g., 3, 4, or 5 or more secondaryelectrodes), the plurality forming an array. In some embodiments, thearray comprises: a leading electrode positioned on the tissue adjacentthe primary electrode; a trailing electrode positioned on the tissueremote from the primary electrode (e.g., about 1 to 4 centimeters fromthe primary electrode; and optionally, one or more (e.g., from 1 to 5)intervening electrodes positioned on the tissue between the leadingelectrode and the trailing electrode.

In some embodiments, the primary electrical stimulus has a duration offrom 0.1 or 0.2 seconds to 1 or 2 seconds (preferably about one halfsecond).

In some embodiments, the secondary electrical stimulus has a duration offrom 0.1 or 0.2 seconds to 1 or 2 seconds (preferably about one halfsecond).

In some embodiments, the excitable tissue is peripheral nerve tissue(e.g., a motor nerve; a sensory nerve).

In some embodiments, the excitable tissue is injured.

A further aspect of the invention is a tissue stimulator for use incarrying out a method as described herein, and/or configured to carryout a method as described herein. Such a tissue stimulator may generallycomprise a power supply, a primary electrode, at least one secondaryelectrode, and a controller configured for carrying out a method asdescribed herein. In some embodiments, the at least one secondaryelectrode comprises a plurality of secondary electrodes.

A further aspect of the invention is the use of a first stimulus and asecond stimulus as described herein for carrying out a method asdescribed herein.

The present invention is readily distinguished from prior art such asU.S. Pat. No. 7,689,289 to King et al. and U.S. Pat. No. 6,473,653 toSchallhorn et al. King et al. reconfigures an excitation pattern byapplying two spatially distinguished sub-threshold electrical stimuli.These stimuli create a superposition of local electric fields and induceadditional supra-threshold excitation zones. In Schallhorn et al.,reconfiguration of excitation pattern is achieved by a controlledactivation of multiple spatially distributed stimulation sites combinedin one lead. Both are simply dealing with reconfiguring of stationaryexcitation patterns using multiple spatially distributed periodicstimuli. Neither address dynamic features of the nerve action potentialand in no way relate to stabilization of its propagation.

The present invention is explained in greater detail in the drawingsherein and the specification set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic limb placement of the external (percutaneous)control unit (C) wirelessly connected with an implantable receiver (R)which simultaneously activates multiple spatially distributedstimulation sites in the implantable electrode (E)

FIG. 1B is a block diagram of stabilization of marginally stable actionpotential propagation in the nerve using a set of additional spatiallydistributed sub-threshold stimuli (S).

FIG. 2 depicts the equations used in a simplified mathematical model ofpropagation of non-linear resonant excitation waves. It also depicts atypical propagating action potential.

FIG. 3 Spatial configuration of primary and secondary stimulation sites.

FIG. 4 Phase portrait of the system with and without additionalexcitation from the secondary sources of stimulation.

FIG. 5 Dependence of action potential duration T_(AP) for differentamplitudes of secondary stimuli.

FIG. 6 Action potential resonant locking mechanism at differentfrequencies and amplitudes of the secondary stimuli.

FIG. 7 Dependence of a minimal stable action potential duration onfrequency ratio for different amplitudes of secondary stimuli.

FIG. 8 Dependence of action potential duration of a primary excitationon period on period of primary stimulations for different values ofamplitudes of secondary stimuli.

FIG. 9 Dependence of a minimal stable action potential duration on anumber of secondary sources of stimulation.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention now will be described more fully hereinafter withreference to the accompanying figures, in which embodiments of theinvention are shown. This invention may, however, be embodied in manyalternate forms and should not be construed as limited to theembodiments set forth herein. Like numbers refer to like elementsthroughout the description of the figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andthis specification and will not be interpreted in an idealized or overlyformal sense unless expressly so defined herein.

The present invention may be embodied as systems, methods, and/orcomputer program products. Accordingly, the present invention may beembodied in hardware and/or in software (including firmware, residentsoftware, micro-code, etc.). Furthermore, the present invention may takethe form of a computer program product on a computer-usable orcomputer-readable storage medium having computer-usable orcomputer-readable program code embodied in the medium for use by or inconnection with an instruction execution system. In the context of thisdocument, a computer-usable or computer-readable medium may be anymedium that can contain, store, communicate, propagate, or transport theprogram for use by or in connection with the instruction executionsystem, apparatus, or device.

The computer-usable or computer-readable medium may be, for example butnot limited to, an electronic, magnetic, electromagnetic, infrared, orsemiconductor system, apparatus, or device. More specific examples (anonexhaustive list) of the computer-readable medium would include thefollowing: an electrical connection having one or more wires, a portablecomputer diskette, a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flashmemory), and a portable compact disc read-only memory (CD-ROM).

The present invention may be described below with reference to blockdiagrams and/or flowchart illustrations of devices, methods and computerprogram products according to embodiments of the invention. It is to beunderstood that the functions/acts noted in the blocks may occur out ofthe order noted in the operational illustrations. For example, twoblocks shown in succession may in fact be executed substantiallyconcurrently or the blocks may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

Applicants specifically intend that the disclosures of all United Statespatent references cited herein be incorporated by reference herein intheir entirety.

“Action potential” is a consecutive rise and fall of the individualcellular transmembrane potential difference which results from cyclicchanges in concentration of ions between extra- and intra-cellularspaces.

“Propagation” of action potential is a diffusive transfer of actionpotentials from one cellular membrane to another which occurs due tochanges of concentration of extra-cellular ions between the neighboringcellular membranes.

“Excitable tissue” as used herein includes muscle tissue (e.g., skeletalmuscle, smooth muscle) and nerve tissue (e.g., peripheral motor nerves;peripheral sensory nerves). The tissue is typically mammalian and in apreferred embodiment is human. The tissue may be in vitro, or in vivo ina subject.

“Subject” as used herein is, in general, male or female human subjectsof any suitable age, including juvenile, adolescent, adult, andgeriatric subjects.

“Injured” as used herein with respect to an excitable tissue refers to atissue that has sustained a chemical injury (chemical burn,toxicological injury), mechanical injury (crushed, lacerated, fully orpartially severed) or the like, such that propagation of a conductedsignal therein is impaired.

“Electrical stimulus” as used herein typically refers to a waveformelectrical stimulus: That is, an electrical stimulus having a repeatingseries of peaks of generally uniform spacing or time intervaltherebetween, which time interval can be expressed as a frequency inHertz. Any suitable waveform shape can be used, including but notlimited to square, ramped, sawtooth, sine, logarithmic, exponential, andcombinations thereof. The waveform shapes can be modulated, truncated,cut, rectified, etc., as is known in the art. Primary and secondaryelectrical stimuli as described herein can have the same, or different,waveform shapes.

“Concurrently” as used herein with respect to primary and secondaryelectrical stimuli means at the same time, or at least partiallyoverlapping in time.

The present invention overcomes limitations of the conventional FESmethod and implements a novel technique of applying distributed sequenceof secondary sub-threshold stimuli resonant with a frequency of primarystimulations. The present invention is based on the discovery that suchnon-linear resonant frequency locking stabilizes propagation at higherprimary stimulation frequencies, thus, facilitating conditions for moreeffective restoration of paralyzed muscles, and/or the treatment of painsuch as neuropathic pain arising from injured sensory nerves. Thismethod can be especially instrumental in tissues with severely impairedconductivity where propagation of a primary wave alone without resonantsecondary stimuli would be completely blocked.

In one non-limiting illustrative embodiment, the present inventionprovides a method of stimulating excitable tissue by resonant spatiallydistributed stimuli to provide greater stability of propagation of aprimary excitation wave. The method comprises the steps of:

-   -   (a) determining if the primary rhythm contains a propagation        block at a particular primary stimulation frequency;    -   (b) if yes, applying additional sub-threshold stimuli at a        secondary stimulation frequency after the first primary        wavefornt passes the last electrode in the chain of secondary        stimulation sites;    -   (c) continuing sufficient number of primary and secondary        stimulation sequencies and rechecking for propagation block;    -   (d) adjusting the number and frequency of secondary stimuli if a        propagation block is still present;    -   (e) repeating steps (b)-(d) until a resonant frequency locking        is achieved and each primary wave results from a single        corresponding primary stimulation without intermediate        propagation blocks.

An illustrative system for implementing is depicted in FIG. 1. Ingeneral, the system comprises:

-   -   (a) implantable device with a primary electrode and a grid of        secondary electrodes which together form a one-dimensional array        of stimulation sites;    -   (b) percutaneous receiver/stimulator    -   (c) external controller/transmitter

The system has four major functions. The first one is using primaryelectrode to deliver over-threshold stimuli to initiate a chain ofprimary excitation waves. The second one is using an electrode array andreceiver/stimulator to detect conduction blocks that prevent the primaryexcitation wave to propagate between any pair of successive stimulationsites. The third function is using controller/transmitter to transmit acorresponding signal and adjust stimulation parameters accordingly if aconduction block has been detected. The fourth function is usingreceiver/stimulator to deliver secondary sub-threshold stimulationpulses, which stabilize propagation of a marginally stable primaryexcitation wave.

The magnitude of stimulation parameters and a number of stimulationsites activated during such a feedback process can be adjusted eitherautomatically or using manual programming of an external controller.

In some embodiments, the present invention accordingly provides atechnique for restoration and training paralyzed muscles in patient whenpropagation of neural impulses is marginally stable or completelyblocked.

In some embodiments, the present invention provides a technique forrestoration and training paralyzed muscles in patient at higherfunctional stimulation frequencies.

In still other embodiments, the present invention provides a techniquefor restoration and training paralyzed muscles in patients with severedamage of neurological tissues.

Those skilled in the art will appreciate numerous additional embodimentsof the present invention, based on known systems and methods andvariations thereof that will be readily apparent to those skilled in theart. See, e.g., U.S. Pat. Nos. 3,662,758; 3,727,616; 4,524,774;6,473,653; and 7,689,289; see also P. Peckahm and J. Knutson, Anna Rev.Biomed. Eng. 7: 327-60 (2005). Accordingly, the foregoing is set forthfor illustrative purposes only, and is not to be construed as limitingthereof.

The controller (typically implemented as circuitry and/or software) canbe provided in a housing, such as sealed housing suitable forimplantation if desired, or a housing for carrying on or associationwith a patient (similar to a TENS unit). The power supply (e.g., abattery) can be contained within the housing or located separatelytherefrom. While the controller is described in combination with theelectrodes above, it will be appreciated that the controller may also beprovided as a subcombination, with connectors (e.g., mounted on thehousing and in turn connected to the controller circuitry) provided forconnecting removable or disposable electrodes thereto.

The present invention is explained in greater detail in the followingnon-limiting Examples.

EXAMPLE 1 Simulation of Propagation of Excitation Waves inOne-Dimensional Nerve Tissue

This example was carried out to simulate a practical realization of thesystem shown in FIG. 1A. FIG. 1B visualizes propagation of excitationwaves which results from a superposition of main (primary) pulse andspatially distributed secondary stimulations applied to one-dimensionalexcitable cable. For our simulations we implemented rate dependentmodification of analytically solvable two variableChernyak-Starobin-Cohen (CSC) reaction diffusion model [11-13] asdepicted in FIG. 2A. Equations (1),(2) describe the process ofpropagation of excitation in an one-dimensional excitable cable offinite length with linear dependence of excitation threshold (memory) onperiod of primary stimulations [14].

In this model, functions u(x, t) and v(x, t) are the membrane potentialand slow recovery current, respectively. Parameter v_(r) is the criticalexcitation threshold necessary to initiate the propagation of an actionpotential through the cable. Another parameter of the model, ε, reflectsthe two order-of-magnitude difference in time constants between the fastexcitation and slow recovery processes.

According to the method of the present invention, a set of primary andsecondary excitation sources formed a spatially distributed array ofindependent stimulation sites. The first site near the cable left endwas a primary stimulation site. Over-threshold stimuli P(x, t) appliedat this site initiated a sequence of primary excitation waves whichpropagated through the cable from left to right with a period T₀ andamplitude A₀. Secondary sub-threshold stimuli S_(i)(x_(i),t) wereapplied at other stimulation sites with a period T and amplitude A,respectively. Secondary stimuli did not initiated additional waves, butprovided additional charge to accelerate the propagation of primarywaves.

If the first primary stimulus applied to a cable at rest(v(x,t=0)=v_(r)) has a sufficient amplitude to overcome the excitationthreshold v_(r), it will initiate action potential which will propagatealong the cable. During the depolarization phase the magnitude of actionpotential, u, rises from its equilibrium state (u(x, t=0)=0) to thevalue of unity, while v rises from its resting value of v_(r). Theintersection between the values of u and v defines the begining of theaction potential. During the repolarization phase (refractory period)both u and v decrease from their maximum values toward correspondingresting values. This results in a second intersection between u and vnear which marks the end of the action potential duration (FIG. 2B). Atany particular spatial point of interest the action potential duration,T_(AP), was computed as the time interval between successiveintersections of u and v. Action potential computations were performedafter delivering 80 primary stimuli to ensure that all correspondingmeasurements reflected steady state values of T_(AP).

In general, there may be n secondary sources of stimulation,S_(i)(x_(i),t), which can be located at any point of the cable (FIG. 3).In our simulations we used six equidistant secondary sites locatedbetween x=40Δx and x=140Δx. The primary stimuli P(x,t) (red dotted line)were applied at every point between x=2Δx and x=15Δx. Here Δx=0.23 is aspatial grid interval. The number of spatial nodes was equal to 150. Forall simulations model parameters were equal to ε=0.1,ζ=1.1,λ=0.4,A₀=1.4.

Rate dependent excitation threshold v_(r)=α−βT₀ was determined by twoconstants 0.31 and 0.0025 for α and β, respectively. Secondarystimulations were simultaneously initiated at all six secondary sitesafter the first primary wavefront (intersection between u and v) reachedthe end of the cable. A phase shift between the onset of primary andsecondary stimuli was determined by the value of primary stimulationperiod.

EXAMPLE 2 Phase Portraits at Marginal Stability of Propagation with andwithout Additional Stimuli

This example was carried out to ascertain a principle of stabilizationof excitation wave using additional sub-threshold stimulations. Atsufficiently long periods of stimulation a slow recovery current v hasenough time to reach its equilibrium state v_(r) before the nextstimulus is applied to the cable. However, at shorter periods v does notcompletely recover and its minimum value, v_(min), is still greater thanthe excitation threshold v_(r) (FIG. 4A). Below a certain value ofprimary stimulation period T_(end) recovery current v becomes so muchhigher than v_(r), that the next primary stimulus may not result ininitiation of a complete action potential. The latest may lead toinstabilities that can occur in the form of alternating actionpotentials (alternans) or complex M:N (M>N) conduction blocks (FIG. 4B).

The present invention demonstrates that such instabilities can bepreempted by applying additional sub-threshold spatially distributedstimuli. FIG. 4C (dashed line) shows a phase portrait of the solution ofEqs. (1)-(2) in the absence of secondary excitations for T_(end)=30 andv_(r)=0.24. In this case propagation of excitation wave is at a marginof stability and therefore countour of u significantly deviates from itsN-shaped nullcline shown in black. Further decrease in primarystimulation period below the critical value T₀*<T_(end) results hiconduction blocks as depicted in FIG. 4B. On the contrary, FIG. 4C(solid line) demonstrates that propagation can be stabilized, if thesecondary stimuli with sub-threshold amplitudes A=0.1A₀ and periodsT=T₀* are applied in addition to primary stimulations.

EXAMPLE 3 Dependence of Safely Margin of Stability of Propagation onAmplitude of Secondary Stimuli

This example was carried out to demonstrate a resonant nature ofdependence of action potential duration, T_(AP), on amplitude ofsecondary stimuli. Period of primary and secondary stimulations were thesame (T₀=30, v_(r)=0.24) while the amplitude A varied from 0 to 0.2A₀.Action potential duration was measured away from the site of a primarysource at the mid point of the cable x=L/2. Dependence of actionpotential duration on A revealed a typical resonant behavior, therebyT_(AP) decreased for amplitudes smaller and greater than resonant valueof 0.0025A₀ (FIG. 5).

EXAMPLE 4 Resonant Action Potential Locking for Different Amplitudes andFrequencies of Secondary Stimuli

This example was carried out to demonstrate the dependence of resonantaction potential locking on amplitude and frequency of secondarystimulations at a constant frequency of primary stimuli

$\begin{matrix}{{F_{0} = \frac{1}{T_{0}}},{T_{0} = 30.}} & \left( {{FIG}.\mspace{14mu} 6} \right)\end{matrix}$

It has been observed that locking of action potential (1:1 response) ateach given secondary stimulation frequency F occured for A which weregreater than certain critical values shown by a curve marked withsquares. At lower values of A, action potentials exhibited more complexresponses encircled by a curve marked with circles (M:N, M>1). Belowthis curve there was no locking and the cable responded only to primarystimulations. An amplitude-frequency locking boundary was assymetricalwith respect to frequency of primary stimulations, thereby the amplitudemargin at F<F₀ was higher than at F>F₀.

EXAMPLE 5 Dependence of the Minimal Period T_(end) of Primary StableWaves on Frequency Ratio

$\frac{F}{F_{0}}$for Different Values of A

This example was carried out to demonstrate the increase of safetymargin of stability of primary waves at higher amplitudes of secondarystimuli. FIG. 7 shows that for higher amplitudes of secondary stimuliregardless the value of secondary-primary frequency ratio stable 1:1primary responses occur at significantly lower values of T_(end). Theincrease of safety margin of stability is more substantial as theamplitudes of secondary stimuli increase from A=0.07 (curve withcircles) to A=0.14 (curve with squares), and the minimal value ofT_(end) decreases by more than 30% regardless of frequency ratio. Forboth curves the values of T_(end) are significantly smaller than thatequal to 30 in the absence of secondary stimuli.

EXAMPLE 6 Steady State Restitution Dependences T_(AP) on Period ofSecondary Stimulations T for Different Values of Amplitudes of SecondaryStimuli

This example was carried out to extend results from the example 5 anddemonstrate stabilization of propagation of primary waves in a widerange of stimulation periods T (FIG. 8). In this example values ofperiods of secondary stimulations T were equal to a primary stimulationperiod T₀. FIG. 8 shows that propagation of primary waves is more stablein the presence of secondary stimulations for different values of T.Indeed, safety margin of stability of propagation significantlyincreases for higher amplitudes and allows more stable propagation forlower T_(AP) and lower minimal values of T_(end).

EXAMPLE 7 Dependence of T_(end) on Different Number of Secondary Sources

This example was carried out to determine the optimal number ofsecondary stimulation sources. The dependence of T_(end) on number ofsecondary sources was determined for n ranging from 2 to 8 for threesets of rate dependence parameters α and β (FIG. 9). The amplitude ofsecondary stimuli was constant and equal to 0.1A₀. The value T_(end)decreased for higher n and saturated beyond a certain number at each setof parameters α and β.

REFERENCES

-   1. C. Veraat, W. M. Grill, T. Mortimer. Selective control of muscle    activation with a multipolar nerve cuff electrode. IEEE Transactions    on Biomedical Engineering 40 (1993): 640-653-   2. R. A. Thorsen, E. Occhi, S. Boccardi et. al. Functional    electrical stimulation reinforced tenodesis effect controlled.    Journal of Rehabilitation Research & Development 43 (2006): 247-256-   3. K. Ambrose, M. A. Wilmarth Functional electrical stimulation for    treatment of chronic foot drop due to an incomplete sacral nerve    root lesion: A case study. The Foot and Ankle Online Journal 3, no.    8 (2010)-   4. E. C. Tuday, K. S. Olree, K. W. Horch Differential activation of    nerve fibers with magnetic stimulation in humans. BMC Neuroscience 7    (2006): 58-64-   5. P. H. Peckham, J. S. Knutson Functional electrical stimulation    for neuromuscular applications Anna Rev. Biomed. Eng 7 (2005):    327-360-   6. S. Trier, T. Vrabec, J. Weisgarber Using functional electrical    stimulation to restore movement to individuals with neuromuscular    disabilities. Matlab Digest, Academic Edition (2010)    www.mathworks.com-   7. S. M. Goetz, R. T. Stone, W. W. Ball et. al. Stimulation    templates for programming a stimulation lead with complex electrode    array geometry. U.S. Pat. No. 7,676,273 (2010)-   8. S. M. Goetz, R. T. Stone, W. W. Ball et. al. Programming    interface with an unwrapped 2D view of a stimulation lead with    complex electrode array geometry. U.S. Pat. No. 7,657,319 (2010)-   9. M. T. Gerber Electrical stimulation lead with conformable array    of electrodes. U.S. Pat. No. 7,769,472 (2010)-   10. A. Dar and R. H. Nathan Scanning electrode system for a    neuroprosthesis. U.S. Pat. No. 7,149,582 (2006)-   11. Y. B. Chernyak, J. M. Starobin, R. Cohen Class of exactly    solvable models of excitable media. Physical Review Letters 80    (1998): 5675-5678.-   12. Y. B. Chernyak, J. M. Starobin, R. J. Cohen, Where do dispersion    curves end? A basic question in theory of excitable media. Phys.    Rev. E 58 (1998): 4108-4111-   13. I. B. Schwartz, I. Triandaf, J. M. Starobin et al. Origin of    quasiperiodic dynamics in excitable media, Phys. Rev. E, 61 (2000):    7208-7211-   14. J. M. Starobin, C. P. Danford, V. Varadarajan et al. Critical    scale of propagation influences dynamics of waves in a model of    excitable medium, Nonlinear Biomedical Physics 2009, 3:4

The foregoing is illustrative of the present invention, and is not to beconstrued as limiting thereof. The invention is defined by the followingclaims, with equivalents of the claims to be included therein.

We claim:
 1. In a method of stimulating an excitable tissue with aprimary electrical stimulus through a primary electrode at a primarystimulation frequency, to produce a propagating action potential in saidexcitable tissue, the improvement comprising: concurrently stimulatingsaid excitable tissue with a secondary electrical stimulus through atleast one secondary electrode at a secondary stimulation frequency;wherein said primary and secondary stimulation frequencies are differentfrom one another; and wherein said secondary electrical stimulus has anamplitude not more than one third that of said primary electricalstimulus; so that propagation of said action potential in said excitabletissue is enhanced.
 2. The method of claim 1, wherein said primarystimulation frequency is from 1 Hertz up to 20 Hertz.
 3. The method ofclaim 1, wherein said secondary stimulation frequency is from 1 to 15percent greater than, or 1 to 15 percent less than, said primarystimulation frequency.
 4. The method of claim 1, wherein secondaryelectrical stimulus has an amplitude not more than one tenth that ofsaid primary electrical stimulus.
 5. The method of claims 1, whereinsaid at least one secondary electrode comprises a plurality of secondaryelectrodes, said plurality forming an array.
 6. The method of claim 1,wherein said array comprises: a leading electrode positioned on saidtissue adjacent said primary electrode; a trailing electrode positionedon said tissue remote from said primary electrode; and optionally, oneor more intervening electrodes positioned on said tissue between saidleading electrode and said trailing electrode.
 7. The method of claim 1,wherein said primary electrical stimulus has a duration of from 0.1seconds to 2 seconds.
 8. The method of claim 1, wherein said secondaryelectrical stimulus has a duration of from 0.1 second to 2 seconds. 9.The method of claim 1, wherein said excitable tissue is peripheral nervetissue.
 10. The method of claim 1, wherein said excitable tissue isinjured.
 11. A tissue stimulator comprising: (a) a primary electrodearray for stimulating an excitable tissue; (b) at least one secondaryelectrode for concurrently stimulating said excitable tissue; and (c) acontroller operatively associated with said primary electrode array andsaid at least one secondary electrode, with (i) said controllerconfigured to stimulate said excitable tissue through said primaryelectrode array with a primary electrical stimulus at a primarystimulation frequency, to produce a propagating action potential in saidexcitable tissue; and with (ii) said controller configured toconcurrently stimulate said excitable tissue through said at least onesecondary electrode with a secondary electrical stimulus at a secondarystimulation frequency; said controller further configured so that saidprimary and secondary stimulation frequencies are different from oneanother; and wherein said secondary electrical stimulus has an amplitudenot more than one third that of said primary electrical stimulus; sothat propagation of said action potential in said excitable tissue isenhanced.
 12. The tissue stimulator of claim 11, wherein said primarystimulation frequency is from 1 Hertz up to 12 Hertz.
 13. The tissuestimulator of claim 11, wherein said secondary stimulation frequency isfrom 1 to 15 percent greater than, or 1 to 15 percent less than, saidprimary stimulation frequency.
 14. The tissue stimulator of claim 11,wherein secondary electrical stimulus has an amplitude not more than onetenth that of said primary electrical stimulus.
 15. The tissuestimulator of claim 11, wherein said at least one secondary electrodecomprises a plurality of secondary electrodes.
 16. The tissue stimulatorof claim 11, wherein said array comprises: a leading electrodepositioned on said tissue adjacent said primary electrode; a trailingelectrode positioned on said tissue remote from said primary; andoptionally, one or more intervening electrodes positioned on said tissuebetween said leading electrode and said trailing electrode.
 17. Thetissue stimulator of claim 11, wherein said primary electrical stimulushas a duration of from 0.1 seconds to 2 seconds.
 18. The tissuestimulator of claim 11, wherein said secondary electrical stimulus has aduration of from 0.1 seconds to 2 seconds.
 19. The tissue stimulator ofclaim 11, wherein said excitable tissue is peripheral nerve tissue. 20.The tissue stimulator of claim 11, wherein said excitable tissue isinjured.
 21. The tissue stimulator of claim 11, further comprising apower supply operatively associated with said controller.
 22. The tissuestimulator of claim 11, wherein said at least one secondary electrodecomprises a plurality of secondary electrodes.